2 Answers
2

The number of mutual friends now includes mutual friends whose accounts have been disabled, but these friends are not displayed. Also, if the person's full list of friends is hidden to you, a mutual friend who also has their full list of friends hidden to you will not be displayed as a mutual friend.

In other words, if a mutual friend's friend list is hidden, it will not be show up.
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AnwarNov 11 '13 at 13:25

1

@Anwar: If either of the two people have their Friends visible to you then you will see them in Mutual Friends. Only if both of them have their Friends hidden then you won't see them.
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mark4oNov 11 '13 at 15:59